Hold circuit for a dryer



DCC. 8, 1970 K, D, SAUSBURY ETAL 3,545,096

HOLD CIRCUIT FOR A DRYER Filed April 3, 1968 3 Sheets-Sheet 1 INVENTORS/jz/Zzzzzjy Dec. 8, 1970 K, D, SALlsBURY ETAL HOLD CIRCUIT FOR A DRYER 3Sheets-Sheet 2 Filed April '5, 1968 DCC 3, 1970 K. D. sALlsBuRY ETALHOLD CIRCUIT FOR A DRYER 3 Sheets-Sheet 3 INVIENTORS Filed April s, 1968ATTORNEYS Patented Dec. 8, 1970 3,545,096 HOLD CIRCUIT FOR A DRYER KeithD. Salisbury, Arlington, Mass., and William F. Robandt, St. Joseph,Mich., assignors to Whirlpool Corporation, Benton Harbor, Mich., acorporation of Delaware Filed Apr. 3, 196s, ser. No. 718,432 Int. Cl.F26b 13/10 U.S. Cl. 34-44 7 Claims ABSTRACT F THE DISCLOSURE A holdsystem for a dryer which, upon preselection dries articles to a presetmoisture level and allows the dryer to continue to operate withoutfurther drying until the operator restarts the heater to complete thedrying cycle.

CROSS rREFERENCE TO RELATED APPLICATION The hold circuit of thisinvention is applicable to dryers such as described in the applicationentitled Tumble Pattern Sensor and Method of Drying, invented by WilliamF. Robant and assigned to the assignee of this invention Ser. No.714,581, `filed Mar. 20, 1968.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates in general to dryers, as for example clothes dryers, and inparticular to a novel control circuit for turning off the heater andcontinuing to operate the dryer in a holding mode until the useractuates the control system to complete the drying cycle.

Description of the prior art U.S. Pat. 2,885,789 arrests operation of adryer when V clothing reaches a damp dry condition in response to asignal from a pair of temperature sensing means. However, no provisionis made to terminate heat input simultaneously with a reduction in drumspeed and no hold circuitry is provided to re-initiate dryer operation.

SUMMARY In accordance with this invention a hold circuit is provided fora dryer which allows articles being dried to be held at a predeterminedmoisture content until the operator desires that the cycle be completed.The hold circuit can be incorporated in a control arrangement wherebyspeed of the driving motor is regulated as a function of variations inthe tumble pattern although the present invention has utility in otherforms of dryer circuitry.

BRIEF DESCRIPTION OF THEI DRAWINGS DESCRIPTION O=F THE PREFERREDEMBODIMENT FIGS. 1 and 2 are schematic views of a clothes dryer 11 whichhas a cabinet 12 with 'a door I13. A control panel 15 is attached to thetop of the cabinet 12 and has a moisture control lknob 14. A startbutton 16, a hold botton -81 and a finish button 82 are also mounted onthe control panel '15. As best shown in PIG. 3 the knob 14 has a pointer17 that may be set against indicia 20 to control the shut-off point ofthe dryer.

FIGS. Z and 4 illustrate a stationary bulkhead 1'8 and the drum 19 ofthe dryer. A motor 21 is coupled to the drum '19 by belt 2.2 to driveit.

An inlet orifice 23 and an outlet oritce 24 are formed in the bulkhead`18. A heater 10 is mounted in the duct (not shown) leading to the inletorifice 23 and a blower motor 26 with a suitable impeller is mounted inthe duct (not shown) leading from the outlet orilice 24 to draw airheated by heater 10 through the dryer. Although IFIG. 4 illustrates atotally stationary bulkhead, the invention may be used where only aportion of the bulkhead is stationary.

As shown in FIG. 4, the outlet orifice 24 is generally above center andto one side of the dryer and arrows X, Y, and Z illustrate various pathswhich articles in the dryer can ta'ke. Arrow X shows a path near the topof the outlet orice 24 and results when the drum speed is relativelyhigh. Arrow Y passes generally over the center of the outlet orifice 24and results from a slightly lower drum speed. Arrow Z illustrates a pathbelo'w the orifice 24 and corresponds to a drum speed which is too slow.From the standpoint of heat transfer to the wet clothing, which ofcourse directly determines the drying rate, a band between arrows X andY ygives the optimum tumbling pattern within the drum. Thus, to maintainan optimum tumbling speed, the drum speed is continuously variedthroughout the drying cycle so clothing always follows the path betweenarrows X and Y in tumbling through the heated air drawn through thedrum. An additional benefit of the control system is that as the drumspeed is continuously varied to force clothing to follow the abovedefined trajectory, the changing speed breaks up any regular tumblingpattern the clothing might establish thereby causing it to ball Thisballing effect is highly detrimental to proper drying since the clothingon the inside of the ball is not exposed to heated air and comes outdamp 'while the clothing on the fringes of the load is over-dried.

In order to sense when the clothing is following the trajectory betweenarrows X and Y, a pair of thermistors 28 and 29 are located immediatelybehind the air outlet 24 through which air exits from the drum. Ifclothing follows the arrow Y, heated air must pass through damp clothingbefore going out the portion of the grill in which thermistor 28 islocated. However, heated air does not go through the clothing loadbefore passing by thermistor 29. If clothing follows the arrow Ythermistor 28 runs about 10 F. cooler than thermistor 29. Thus, tocontrol the drum speed so that the clothing follows a path betweenarrows X and Y, a speed control circuit for the motor receives inputsfrom thermistors 28 and 29 and continuously varies the drum speed sothat the temperature differential between thermistors 2 `Sand 29 ismaintained at a maximum.

As the clothing within the drum begins to dry, hot air passing throughthe partially dried clothing no longer is cooled as much as it was atthe beginning of the cycle when the clothing was completely soaked.Thus, near the end of the drying cycle thermistor 28 begins to approachthermistor 29 in temperature. It has been experimentally determined thatthe temperature differential between thermistors 28 and 29 is linearlyrelated to the percent moisture retention of the load, when it followsthe path between arrows X and Y. Thus, by using the temperaturedifferential between the two thermistors, a source of signal for anextremely accurate dry control is obtained.

The housewife sets knob 14 to the desired moisture retention and pressesstart button 16 to start the dryer. If the housewife wishes to hold thearticles being dried at a predetermined moisture level, she pressesbutton 81. When the articles reach the predetermined moisture level theheaters cut ot andthe drum speed drops to approximately 39 r.p.m. The`clothing continues to tumble at the predetermined moisture level untilthe housewife returns anddepresses the nish button 82. The dryer willthen complete the drying cycle and turn oi. This prevents the clothingfrom becoming completely dry prior to the arrival of the housewife atthe dryer. Thus, lshe can remove the clothing from the dryer preciselywhen it terminates which prevents wrinkles from setting in new permanentpress fabrics. v

As best shown in FIGS. a and 5b commonly available A.C. power, as forexample 110 volts, is connected between terminals B and N of the dryer.A 220v volt supply is connected between terminals B and C. A door switch31 is connected to terminal B and is open when the door 13 is open. Whendoor 13 is closed, the switch 31 closes to connect terminal B withcontact 32 of switch 16. Contact 32 is connected to a thermostat switch33 which is in series with a centrifugal switch-34 actuated by motor 21when it comes up to speed, a heater switch 36 and the heater 10. Theother side of the heater is connected to terminal C.

A second terminal 37 of Switch 16 is connected to hold relay 38 whichhas its other side connected to terminal N. The relay 38 has an armaturewhich moves linkages 35 to control switches 36 and 39.

Switch 39 has one contact connected to contact 32 of switch 16. A secondcontact 41 is connected to a lead 42. A cool-down thermostat 43 isconnected between contact32 and lead 42.

A normally closed switch 44 has one contact connected to contact 41 anda second contact 46 connected to one side of relay 38.

A hold switch 86 comprises the hold button 81 and a pair of contacts 87and 88. Contact 87 is connected to a contact 89 of switch 16. Lead 91connects contact 88 to one side of relay 92 which has its other sideconnected to point N. The relay 92, when energized, closes switches 93and 94 by linkages 96 and 97. Switch 93 is connected between contact 87and relay 92 and provides holding current for relay 92 when button 81 isreleased.

The motor 21 has a run winding 47 and a start winding 48. First sides ofthese windings are connected to lead 42. The other side of winding 48 isconnected to a centrifugal Switch 49 which opens when the motor comes upto speed and has its other -side connected to a capicitor C1. The otherside of capacitor C1 is connected to terminal N. A condensor C2 isconnected from terminal N to winding 48. A triac 51 has its anodeconnected to winding 47 and its cathode connected to terminal N. A gateelectrode 52 is connected to a resistor R1 and the anode of a diode D1.The cathode of diode D( is connected to the secondary 53 of transformerT1. The other sides of secondary 53 and resistor R1 are connected toterminal N.

Al full wave rectier bridge 154 comprising diodes D2, D3, D4 and D5 isconnected between terminal N and a resistor R3. The other side ofresistor R3 is connected to lead 42. A resistor R2 is connected from thejunction point of diodes D2 and D3 to a lead 54. A lead 56 is connectedto the junction of diodes D4 and D5. A Zener diode D0 is connectedbetween leads 54 and 56. A capacitor C3 is connected across Zener diodeD5.

The primary 57 of transformer T1 is connected to line 54 and to thecollector of a transistor T10. The emitter of transistor T10 isconnected to a resistor which has its other side connected to lead 56. Aresistor R5, a unijunction transistor T0 and a resistor R3 are connectedin series between lead 54 and 56.

A variable resistor R2 and a capicitor C11 are connected between lines54 and 56. The emitter of transistor T0 is connected to the pointbetween resistor R1 and capacitor C11. A. diode D7 has its cathode alsoconnected to this point and it-s anode connected to resistors R0, R0 andthe collector of transistor T2. The other side of resistor R8 isconnected to lead 54. A resistor R11 is connected between the emitter oftransistor T2 and lead 56. The other side of resistor R0 is connected tothe base of transistor T2.

A resistor R10 is also connected to the base of transistor T2 and topoint H. A capacitor C4 is connected between point H and lead 56.

Refer to FIG. 5b wherein points D, K,.E, H, F and G correspond to thesame points in FIG. 5a.

The motor 21 has an output shaft 58 which drives a tachometer 61. Theytachometer 61 provides an A.C. input voltage to a bridge circuit 62.The tachometer 61 has an output coil 63 connected across points M 'and Oof bridge 62. Resistor R13 is connected between points Q and O andresi-Stor R14 is connected between points O and P. Thermistor 28 isconnected between points M and P and thermistor 29 is connected betweenpoints M and Q. A resistor R12 is connected between point Q and lead 56.A wiper contact 65 engages resistor R12 and is connected to the anode ofdiode D14. The cathode of diode D14 is connected to resistor R10.

A resi-Stor R15 and capacitor C12 are connected in series between pointQ and the base of a transistor T3.

A transformer T20 has its primary 64 connected between leads 42 and 56.The secondary 66 of transformer T20 is connected across a full waverectifier bridge 67 having diodes D3, D3, D10 and D11. A resistor R10 isconnected from the junction of diodes D3 and D10 to lead 68. A capacitorC5 and Zener diode D12 are connected in parallel between leads 68 and69.

Resistors R11 and R13 are connected in series across leads 68 and 69. Acapacitor C3 is connected across resistor R10. The junction pointbetween resistors R11 and R13 is connected to the base of transistor T3,and a resistor R20 is connected between the emitter of transistor T3 andlead 69.

Resistor R21 and capacitor C0 are connected in series between leads 68and 69. Resistor R22 is connected between the emitter of transistor T3and the junction point between resistor R21 and capacitor C3. CapacitorC1 is connected across R21.

A resistor R23, transistor T5 and a capacitor C10 are connected inseries between leads 68 and 69. A resistor R25 is connected acrosscapacitor C10.

A Zener diode D13 and resistor R24 are connected in series between thecollector of transistor T5 and the point between resistor R21 andcapacitor C0. A resistor R20 is connected between lead 68 and the baseof transistor T5.

A capacitor C0, transistor T4 and relay coil 71 are connected in seriesbetween leads 68 and 69. The base of transistor T4 is connected tocapacitor C10.

The relay coil 71 controls a linkage 59. A wiper arm 72 is connected tothe emitter of transistor T4 and forms a part of potentiometer 73. Thisis the moisture control element and knob 14 is connected by shaft 74 tocontrol the position of wiper arm 72. A switch 101 has its movablecontact connected to lead 68 and moves between contacts 102 and 103.Contact 102 is connected to resistor R21 which has its other sideconnected to potentiometer 73. Contact 10'3 is connected to resistor R20which has its other side connected to wiper arm 72. Switch 101 iscontrolled by relay 92 through linkage 96.

In operation, the speed of motor 21 is maintained so that optimum speedof tumbling occurs. The user places clothing or other articles vto bedried in the dryer and sets pointer 17 by knob 14 to the desired finalmoisture content of the articles. This sets Wiper arm 72 ofpotentiometer 73 to a particular resistance.

The door 13 is closed which closes door switch 31 and the start button16 is momentarily depressed to engage contacts 32 and 37. Assume thatthe hold button is not depressed in this example. This supplied power torelay 38 which closes switches 36 and 39. Switch 39 supplies holdingcurrent to relay 38 through normally closed switch 44.

The heater will be energized through normally closed safety thermostat33 and centrifugal switch 34 when the motor 21 comes up to speed.

Switch 39 also provides power on line 42 to a motor speed controlcircuit and a dry control circuit. Normally open thermostat switch 43 isopen unless the temperature in the exhaust stack of the dryer reaches ahigh temperature as, for example, 110 F.

Both the motor speed control and the dryer control receive signals fromthe bridge circuit 62.

The motor 21 starts with normally closed centrifugal switch 49 allowingcurrent to pass through the start winding 48 and start capacitor C. Asthe motor comes up to speed, switch 49 opens and the run winding 47continues to energize the motor.

The tachometer 61 is driven by motor 21 through shaft 58. The tachometerinduces a voltage in winding 63 which varies in frequency and magnitudewith the speed of motor 21.

Bridge 62 includes thermistors 28 and 29 (see FIG. 4 for their mountingpositions) and resistors R13 and R14. Resistor R13 has a lowerresistance than resistor R14. When the dryer starts, the resistance ofthermistors 28 and 29 are the same due to their resistance versustemperature curves being identical. Thus, at the initiation of thecycle, a signal voltage appears at point Q. Also, any time that theresistance of thermistor 28 approaches the resistance of thermistor 29,a signal voltage will appear at point Q because of the initial unbalanceof the bridge. It should be remembered that the voltage magnitude atpoint Q of the bridge 62 depends on two factors, (l) the speed of motor21 and (2) the temperature difference between thermistor 28 and 29.

The signal voltage at point Q is utilized to regulate the speed of motor21 so that the tumbling pattern is maintained at an optimum. This signalcontrols the firing point of triac 51 which regulates power supplied torun winding 47 in the following manner:

Voltage for the motor speed control is supplied by line 42 throughresistor R3. This is full wave rectified by bridge circuit 154. The fullwave rectified A.C. then passes through a current limiting resistor R2and is applied to iilter capacitor C3 whose output voltage is clipped byZener diode D6.

Capacitor C3 has a small capacitance so that it does not maintain a setlevel of ripplefree D.C. voltage on line 54. Instead, this voltage risesand falls, giving a generally clipped full wave signal, as shown bycurve 78 above line 54. This is important because triac 51 must be redat a controlled point in each half cycle of A.C. power and the firingpoint must be synchronized to power on the A.C. line.

Wiper arm 65 is set on resistor R12 to preset a desired voltage. Thisvoltage is half wave rectified by diode D14 and furnished to ltercapacitor C4. The voltage across capacitor C4 is thus proportional tothe signal voltage at point Q.

The voltage on capacitor C4 supplies base drive to transistor T2.

If the signal voltage at point Q is high, transistor T2 will be driveninto saturation and the voltage appearing at its collector will be low.Thus, current passing through resistor R8 each time the voltage rises atpoint Q is shunted to ground through transistor T2 and does not passthrough diode D7 to capacitor C11. All charge current to capacitor C11must pass through resistor R7. Resistor R7 is set such that the voltagelevel on capacitor C11 builds at a slow rate. At a predetermined level,the voltage on capacitor C11 res unijunction transistor T6 and thecharge dumps from capacitor C11 through transistor T6 and resistor R6.This creates a voltage drop across resistor RG which biases switchingtransistor T10 on. A current pulse is then drawn through the primary 57of pulse transformer T1. This generates a pulse across the secondary 53of the pulse transformer T1 and triac 51 is tired through diode D1. Oncetriac 51 fires, power is delivered to the motors run winding 47. Notethat a high magnitude of signal voltage at point Q results in delayedtiring of triac 51 and motor 21 is caused to run at a low speed.

If the output voltage at point Q is low (which is the case when bridge62 is balanced due to thermistor 29 being hotter than thermistor 28thereby insuring that its resistance is lower than thermistor 28, orwhen the motor is rotating slowly causing tachometer 61 to generate alow voltage) capacitor C4 will have a low voltage appearing across it.This biases transistor T2 to a nonconductive state and current passes tocapacitor C11 via resistor R8 and diode D7, as well as through resistorR7. This insures that unijunction transistor T6 is tired early in eachhalf cycle and thus triac 51 fires early in each half cycle. The motor21 now runs at a higher rate of speed due to the increased powersupplied to run winding 47. In summary, with signal voltage low at pointQ, the motor runs at a relatively high rate of speed.

As shown in FIG. 4, it is to be noted that the tumble pattern of theclothing should follow a trajectory lying somewhere between the rows Xand Y. The control circuit of this invention assures this in thefollowing manner. When the drying cycle is initiated, thermistors 28 and29, are, of course, at equal temperatures. This causes the bridge 62 tobe unbalanced and the output at point Q tends to be high due to theeffect of the thermistors. This effect alone would tend to slow themotor down and the clothing would be expected to follow the arrow Z.However, with the clothing following the arrow Z, the motor is rotatingat such a low speed that tachometer 61 applies a very small voltage tobridge 62. Although the thermistor bias tends to slow the motor down,the input to the bridge is so low that no appreciable signal appears atpoint Q. Thus, the motor control circuit applies firing pulses to triac51 early in each half cycle of the A.C. line and motor 21 speeds up. Theclothing is thus not allowed to follow the path of arrow Z simplybecause bridge 62 will be supplied with insufficient voltage to producean output of any magnitude at point Q with the motor rotating at such aslow speed.

Assuming the motor is rotating at a higher speed, which causes theclothing to follow the trajectory of arrow X, both thermistors 28 and 29are covered by the clothing. Thus, their temperature and resistancebecome equal. Due to the different resistance of resistors R13 and R14,a high signal voltage appears at point Q. Also, since motor speed ishigh, the magnitude of tachometer output is high causing the input tothe bridge to be high. Thus, both bridge unbalance and high motor speedcause the output at point Q to be high. Capacitor C4 now charges to ahigh value and saturates transistor T2. This insures that the firingpulse delivered to triac 51 via pulse transformer T1 is delayed in eachhalf cycle of A.C. line voltage, causing motor 21 to slow down. Thus,clothing no longer follows the path of the arrow X.

When the clothing follows the path of arrow Y, thermistor 28 is shieldedfrom hot air passing through air outlet 24 while thermistor 29 receiveshot air. This causes thermistor 29 to heat above thermistor 28 and itsresistance value drops. Bridge 62 now tends toward balance and theoutput voltage at point Q decreases. The charge level on capacitor C4decreases correspondingly and transistor T2 begins to go out ofsaturation. This allows earlier charging of capacitor C11 in each halfcycle of A.C. line voltage and the triac 51 is fired earlier. Thus, themotor speed up. By this action, the clothing is caused to continuouslyoscillate in a band lying between arrows X and Y.

It should be noted that when clothing is lirst placed in the dryer, thedrum speed for an optimum tumble pattern is different than the drumspeed for an optimum tumble pattern near the end of the drying cycleafter the clothing has nearly dried. It has been found that the averagedrum speed must increase as the cycle progresses to keep the clothin-gin the band defined by arrows X and Y. The speed control of thisinvention regulates the average drum speed such that it does increasethroughout the cycle of operation thereby maintaining an optimum tumblepattern throughout the cycle. It is also noted that since the clothingis forced to oscillate in a band between arrows X and Y, any establishedtumbling patterns are broken up and the clothing is always placeddirectly in the path of the heated air. This not only assures minimumdrying time but also assures that the clothing will not ball which wouldlead to overdrying of the outside of the load while the inside remaineddamp.

The dry control portion of the invention is shown in FIG. b. The voltageat point Q is also used to control the termination point of the dryingcycle. Power on line 42 passes through the primary 64 of stepdowntransformer T20 and appears on the secondary 66 at a reduced voltage.Bridge circuit 67 full wave rectifies the output of transformer T20 andapplies it to a filter capacitor C5 via current limiting resistor R15.Zener diode D12 regulates the voltage across capacitor C5 to a DJC.level consistent with the ratings of components within the dry controlcircuitry.

Potentiometer 73 forms part of a voltage divider consisting of a fixedresistor R27, potentiometer 73, and relay coil 71. Thus, the initialsetting of potentiometer wiper 72 determines the emitter voltage oftransistor T4. Transistor T4 is connected in a regenerative fashion withtransistor T5. lf a voltage of magnitude greater than the emittervoltage of transistor T1 appears at the base of transistor T4, it beginsto conduct. This draws current from the base of transistor T5 which alsobegins conduction. As transistor T5 goes into conduction, the base driveto transistor T4 is reinforced and the pair of transistors cascade intoconduction. When these two transistors become conductive, the dryingcycle is terminated as described below.

The time when transistors T5 and T1 cascade into conduction iscontrolled by the signal voltage at point Q which is A.C. coupled to thebase of transistor T2 via a resistor R and capacitor C12. As long as theclothing within the drum is damp, the motor speed control insures thatit oscillates within the band defined by arrows X and Y. When clothingfollows arrow Y, output at point Q is low and the voltage coupled totransistor T5 is correspondingly low. Thus, transistor T5 does notconduct and charge does not pass to capacitor C5 through resistor R22.When clothing follows the path of arrow X, output at point Q causestransistor T2 to conduct and charge begins to accumulate on capacitorC8. However, as long as clothing within the drum is damp, it coolsthermistor 28 when clothing follows arrow Y. This assures that thecharge applied to capacitor C8 while the clothing follows arrow X -isbled away from the capacitor while the clothing follows arrow Y.

This alternate charging and discharging of capacitor C8 does not allowthe voltage at point U to rise to a level exceeding the 'Zener voltageof Zener diode D12 and thus current does not pass through Zener diodeD12 and resistor Rm to circuit point V. This assures that with dampclothing in the dryers drum, transistors T4 and T5 are not cascaded intoconduction. However, as the clothing begins to dry, thermistor 28 is notcooled by clothing following arrow Y and the resistance of the twothermistors begins to equalize. Now the signal voltage at point Qremains at a high level for an extended period of time. This signalvoltage is continuously coupled to transistor T3 and it remainsconductive on a continuous basis. Thus,

the charge level on capacitor C3 builds until it rises above the Zenervoltage of Zener diode D12. Current now passes to circuit point V andtransistors T4 and T5 cascade into conduction. The amount of currentwhich must be supplied at point V to cascade the two transistors isdetermined by the emitter bias on transistor T4 which is, of course,preset by potentiometer 73. This permits the termination point of thedrying cycle to be varied by potentiometer 73 and thus the nal moistureretention of the clothing may be varied.

When transistor T4 and T5 cascade into conduction, relay coil 71 isenergized, thereby opening switch 44 and breaking power to relay coil38. When relay coil 38 drops out, contacts 36 and 39 open. This breakspower to the machines heater 10. However, note that normally opencool-down thermostat 43 has closed during the drying cycle since theexhaust stack temperature has risen. Power continues to be suppliedthrough thermostat 43 to line 42. This keeps motor 21 running until theexhaust stack temperature drops far enough for thermostat 4,3 to open,whereupon the cycle of operation is terminated and the clothes may beremoved from the machine.

If at initiation of the cycle of operation the housewife depresses holdbutton 81 after depressing start button 16, the dryer operates until apredetermined moisture retention is reached (for example, 25%). Thedryer will disconnect power to the heater and drop the drum speed to aslower speed (for example, 39 rpm.) and continue to operate under theseconditions until the housewife returns and presses the finish button 82.The dryer will then reapply power to the heater 10 and will increase thedrum speed until the moisture content set by knob 14 is reached. Thedryer then turns off and the clothes may be removed.

Hold button 81 closes switch 86 which applies power to hold relay 92which closes switches 93, 94 and moves switch 101 so that it engagescontact 1013. Relay switch 93 energizes relay 92 through contact 89,switch 16 and contact 32. Switch 94 bridges thermostat 43 and power issupplied to line 42. Switch 101 puts resistor R28 into the dry controlcircuitry. Resistor R22 assures that relay 71 will be energized when themoisture content reaches the desired hold level (for example 25%). Themotor control and dry control operate as previously described except thecut-olf point for the dry control is determined by resistor R28.

When the articles in the dryer reach the moisture content of 25 forexample, relay 71 is energized. This opens switch 44 and relay 38 dropsout. This opens switches 36 and 39. Power is thus terminated to theheater 10. However, since relay switch 94 is closed due to energizationof relay 92, power continues to be supplied to line 42 indefinitely.Since the heaters have been cut off, the operating characteristics ofthermistors 28 and 29 Slow the motor 21 to a speed of about 39 r.p.m.

When the housewife returns and wishes to complete the drying cycle shepushes finish button 82. This button is connected to switch 16 anddisconnects contacts 32 and 89 and connects contacts 32 and 37. Thisenergizes relay 38 and de-energizes relay 92.

Relay 38 initiates the drying cycle as described under the conditionwhere the hold button had not been depressed and the dryer operatesuntil the moisture content set by knob 14 is reached at which time therelay 71 is energized and the dryer is turned off as previouslyvdescribed.

The hold cycle could also be modied by adding a timer which would moveswitch 16 a preset time after the hold condition had been reached. Thiswould insure that the dryer could not operate indenitely in the holdmode should the housewife forget to initiate termination of the cycle.

Thus, it is seen that this invention provides a hold circuit for a dryerwhich allows a user to selectively hold 9 the articles being dried at apreset moisture level until it is desired to finish the drying cycle.

The principles of the invention explained in connection with thespecific exemplications thereon will suggest many other applications andmodications of the same.

It should be understood that we wish to embody within the scope of thepatent warranted hereon all such modilications as may reasonably andproperly come within the scope of our contribution to the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are delined as follows:

1. A hold circuit for a dryer having a heater and a motor for operatingthe dryer, a motor control circuit, a moisture detecting meansresponsive to a tirst predetermined moisture level and to a secondvariable moisture level, selectively operable control means connected incircuit with the heater and receiving an input from the moisturedetecting means to interrupt the drying cycle by reenergizing the heaterat said first moisture level while the motor continues to be energizedand means operable to reenergize the heater to complete the drying cycleat said second moisture level.

2. A circuit according to claim 1 wherein said motor is a variable speedmotor and the control means is connected in circuit with the motorcontrol circuit to control the speed of the motor to a standard holdspeed.

3. A circuit according to claim 1 wherein the control means comprises ahold switch, a hold relay connected in circuit with the hold switch, amotor energize hold switch connected in circuit with the motor controlcircuit and controlled by the hold relay, a heater control circuit 10including a heater control switch, and the hold relay connected tocontrol the heater control switch.

4. A circuit according to claim 3 having a moisture control circuitforming a portion of the moisture detecting means, normal and holdmoisture setting means in the moisture control circuit, a moistureswitch connected to the normal and hold lmoisture setting means andcontrolled by the hold relay to connect the hold moisture setting meansin circuit when the hold switch has been closed.

5. A circuit according to claim 4 having a nish control, the finishcontrol connected in circuit with the hold relay to de-energize it andconnect the normal moisture setting means in circuit to reenergize theheater.

6. A circuit according to claim 5 wherein the nish switch de-energizesthe hold relay, energizes the heater, opens the motor energize holdswitch, and connects the normal moisture setting means in circuit withthe motor control circuit.

7. A circuit according to claim 6 wherein the control means is connectedin circuit with the heater and the motor control circuit to turn themolf when a predetermined moisture level has been reached after theiinish control has been energized.

References Cited UNITED STATES PATENTS 3,253,347 5/ 1966 Kripke 34-463,286,359 11/1966 Orr et al. 34-53 3,460,267 8/ 1969 Lorenz 34-45 EDWARDI. MICHAEL, Primary Examiner

